In a number of industrial processes carried out under vacuum or in a controlled atmosphere, it is desirable to measure the weight of an object located inside the process chamber and fastened to a movable shaft which passes through a seal to support means located outside the chamber. Friction at the seal between the shaft and the chamber leads to serious errors in weight measurement.
One such industrial process where the need for accurate weight determination is particularly acute is in vacuum consumable electrode arc melting. In a vacuum arc remelt process, an electrode fastened to the end of an axially movable shaft is progressively melted by an arc in a vacuum region for the purpose of driving off impurities in the material of the electrode. This is done several times to purify the material of the electrode and each time it is done, the electrode is shifted by the axially movable shaft within a housing which is evacuated and within which the arc occurs.
To monitor the amount of electrode left on the end of the shaft as the arc melts the electrode, the shaft has a sensor associated with it. This sensor is a load cell which measures the apparent weight of the electrode and shaft. The apparent weight may differ from the actual weight due to the existence of friction at the seal needed to preserve the vacuum or controlled atmosphere within the chamber. The breakaway seal friction in commercial vacuum arc furnaces may be as much as several hundred pounds. By reducing or substantially eliminating the axial component of frictional force associated with the seal between the shaft and the chamber, the apparent weight can be made more nearly equal to the actual weight of the electrode. The present invention as hereinafter described is directed to the elimination of such breakaway friction to thereby reliably provide a more accurate measure of the apparent weight of the electrode.